could anyone tell me if this is worth the effort / expense or does the problem still exist after these modifications?
I now have a much stiffer yard and it has made a lot of difference to the set of the sail in strong breezes. Have you experimented with the luff downhaul and mainsail outhaul? Also, have a look at the lacing to the yard. Mine has gone a bit saggy after Sail Caledonia and is probably going to be replaced.
If you do get a new yard, I suggest that you make your halyard attachment points slightly lower than with your current yard, thus hoisting the yard slightly higher. You could compensate by replacing the downhaul track with a longer version.